The present invention relates to pulse combustion heating and, more particularly, to high fuel energy input pulse combustion burners wherein self-feeding of one or more components of a combustible gaseous mixture is effected with flow metering and immediate combination of the metered flows to enable mixing rate controlled pulse combustion and a large burner turn-down range with adjustment of the fuel/air ratio.
In pulse combustion burners of the Helmholtz type, an oscillating or pulsed flow of combustion gases through the burner is maintained at a frequency determined by burner component geometry and fuel supply characteristics, including the mixing of components thereof. Typically, a combustion chamber of a given size cooperates with a tailpipe or exhaust pipe of specific dimensions to provide explosive combustion cycles, thermal expansion of the combustion gases, and oscillating gas pressures which provide the pulsed flow of combustion gases through the burner.
In order to make the pulse combustion process self-sustaining, the oscillating gas pressures may be used to provide self-feeding of one or more of the components of the combustible gaseous mixture which generally comprise air and a gaseous fuel such as natural gas. It is known to use one-way flapper valves to self-feed air and/or fuel gas to a pulse burner. Such flapper valves include a flexible flapper or diaphragm movably mounted between a valve plate having valve flow openings therein and a backer plate arranged to limit the movement or stroke of the flapper.
The operation and stability of pulse combustion burners are dependent upon the burner geometry and the degree of air and fuel mixing as indicated. These factors also affect the ease of initiating ignition and maintaining substantially complete combustion which is energy-efficient and within emission standards. Accordingly, pulse combustion burners are not readily amenable to operation over a wide turn-down range (maximum BTUH input rate/minimum BTUH input rate). The turn-down range in a typical pulse combustion burner is about 2:1. If the input rate is reduced below a minimum operating value, the process stability self-decays as reduced operating pressures result in correspondingly reduced fuel input rates until burner shut-down occurs. In a related manner, air and/or fuel supply variations may cause significant changes in the operation of the burner, including burner shut-down.
Burners of the type discussed above are mathematically modeled according to acoustic principles and are referred to as "acoustic controlled" hereinafter. In such burners, the air and fuel gas are typically mixed by injection into a mixing chamber along intersecting paths. The mixing chamber dimensions are determined by acoustic operation principles, and the mixing process is not further optimized. Such acoustic design limited mixing generally provides satisfactory mixing and homogeneous combustible gaseous mixtures in relatively low fuel energy input burners having input rates ranging up to about 200,000 BTUH, for example, burners of the size used in residential heating applications.
Even in pulse burner applications having inputs in the range of one to several hundred thousand BTUH, acoustic controlled mixing is not always sufficient to provide a homogeneous combustible gaseous mixture and to achieve efficient burner operation within emission standards over a range of operating conditions. In order to allow for cold start-up conditions and to control emissions of carbon monoxide (unburned fuel) and oxides of nitrogen, U.S. Pat. No. 4,260,361 discloses a multi-stage pulse combustion process wherein fuel gas is radially injected into a flow of air in a suction pipe for fuel-rich combustion in a primary combustion chamber followed by the injection of additional air and lean combustion in a downstream pulsation tube.
In larger sized burners, such as industrial burners having inputs of 700,000 BTUH and higher, acoustic controlled feeding of the combustible gaseous mixture has not been found to provide a homogeneous combustible gaseous mixture and smooth burner operation over a range of conditions, especially if a large burner turn-down range is required. In such burners, failure to completely mix the air and fuel tends to result in incomplete combustion and higher carbon monoxide concentrations in the combustion products. This is believed to result from a reduced flame temperature and burn rate of the air and fuel mixture, the burner operation being characterized by an extended flame length. Burner operation is thus limited by the mixing rate of air and fuel gas. In order to achieve homogeneous air and fuel mixtures in such larger size burners, independently pulsed feeds of air and fuel have been used with computer-controlled variation of the composition of the feed in alternate cycles, complex shaped fuel mixer arrangements and auxiliary combustion chambers as shown in U.S. Pat. No. 4,473,348. In a substantially different approach, U.S. Pat. No. 4,708,635 teaches the series connection of a relatively smaller sized primary pulse burner and a substantially larger sized main pulse burner to provide an integrated combustion process wherein the primary burner provides operating and control characteristics.